Inorganic carbon acquisition in Palmaria palmata (Rhodophyta)
DIC acquisition mechanisms and the use of DIC in photosynthesis were examined in the temperate red macroalga, Palmaria palmata (Linnaeus) O. Kuntze. Photosynthetic O2 evolution was unsaturated at the ambient DIC concentration in seawater. Experiments with inhibitors of the mammalian HCO3 -/Cl- anion exchanger AE1, and the HCO3 - -hydroxylating enzyme carbonic anhydrase (CA), showed that HCO3 - uptake is mediated mainly by a bicarbonate transporter protein. Photosynthesis measured at small increments of DIC addition showed biphasic kinetics, with a hiatus located at 0.625 mM DIC. Inhibitor experiments indicated that extracellular CA is active at low DIC, therefore the biphasic kinetics of photosynthesis may have been caused by the presence of two DIC acquisition mechanisms. Culturing thalli for 6 d in artificial seawater containing 8 mM DIC induced a bicarbonate transporter in the CAuser Chondrus crispus, but no change in activity was detected in P. palmata. Presence of a bicarbonate transporter mechanism was also confirmed by using the polymerase chain reaction. Primers based on the mammalian AE1 gene sequence were used to amplify a 1500bp fragment of bicarbonate transporter gene from genomic DNA extracted from P. palmata and the calcifying microalga, Emiliania huxleyi. Comparison of the algal and mammalian sequences revealed a high amino acid sequence homology, indicating that they encoded structurally and functionally similar proteins. 3 F.F. Blackman’s precept, which implies that carbon limitation does not occur at subsaturating light, was investigated by measuring induced chlorophyll fluorescence at different DIC concentrations and irradiances. Photosynthetic electron transport rate was was stimulated at saturating light, but only slightly at subsaturating light. Thus, Blackman’s theory was upheld. Photosynthesis and bicarbonate transporter activity was monitored in P. palmata growing at St. Margaret’s Bay, Kent, UK. Distinct peaks in photosynthesis occurred in March and October, and coincided with equinoctial tides. Bicarbonate transporter activity was maximal in spring, and decreased during summer. Although it was not possible to show what determined the seasonal patterns of photosynthesis and bicarbonate transporter activity, the possibility that exposure to springwater containing high concentrations of DIC and nitrate is discussed.
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